The invention relates to an apparatus based on the telecentric imaging system for forming an image of a linear zone of an object, the apparatus comprising: a non-telecentric camera consisting of an objective and an image plane formed by a row of photosensitive cells; telecentric imaging means which are located between the objective and the object and which comprise a concave strip mirror, which is substantially parallel with said row of cells and in whose focal plane the aperture of said objective is located, the concave strip mirror and the objective forming jointly telecentrically an image of the object on the row of photosensitive cells; and a light source generating a radiation which is directed to the object. The invention also relates to a method for forming a telecentric image of an opaque object located on an opaque and non-reflective substrate with the telecentric system, in which: diffused light is allowed to be directed to the object over its entire width to be inspected; radiation reflected from the object is collected with the concave strip mirror and is allowed to be rereflected as a bundle of rays from the concave mirror; in said reflective bundle of rays, a camera consisting of an objective and a row of photosensitive cells is disposed with the objective aperture located in the focal plane of the concave strip mirror, so that the concave strip mirror and the objective jointly form a telecentric image of a linear strip of the object on the row of photosensitive cells.
A typical optical monitoring apparatus consists of a radiation source and a camera consisting of an objective and an image plane. The image of the object formed by the objective on the image plane can be monitored and stored e.g. by means of a CCD cell, which converts the image into an electric signal in a conventional manner. A CCD cell consists of photosensitive elements placed in matrix configuration, e.g. 256×256 elements. Then the properties of the camera resembles a conventional photographing camera. However, the matrix of the CCD cell can have e.g. the shape of 1024×1 or 2048×1, whereby the cell elements will be located in one single linear row. A camera comprising a linear CCD cell is commonly called a line camera. For the entire object to be measured, either the object or the camera must be movable relative to one another. The illumination is usually carried out as top illumination.
Conventional camera opticals sees the object differently depending on the location of the object in the measured area. At the optical axis of the objective, i.e. the central area of the object, the object is seen at right angles by the camera, whereas its lateral areas are viewed at oblique angles, which grow as the distance between the camera and the optical axis grows. This is the so-called central perspective. In addition, when the width of e.g. a material to be cut or sawn, such as boards, is monitored, the edges of the cut/sawn zones produce shadows which interfere with the measurement, so that the width measuring system may interpret the shadow as the edge of the strip. In the case of sawing e.g. a log of timber, the distance from the object to the lens also varies as a result of the three-dimensional object, and then the areas of the object which are located at a greater distance from the camera, such as the lateral zones of a log, are seen in a smaller size than the central zone.
Perspective errors can be corrected with the aid of a telecentric objective. Then all the rays emanated from the object will be parallel with the optical axis and all the points of the object are seen in the same plane perspective. In telecentric objectives, the lens closest to the object must have a width at least equal to that of the object, and thus opticals consisting of conventional lenses may become heavy and bulky. In order to provide an apparatus which is at least compact enough to be useful, the problem consists in the high f-number required, involving in turn the use of lenses made of costly special glass and having a strong curvature, i.e. a small radius of curvature. In addition, a plurality of such lenses is needed in one single optical apparatus for adequate correction of the optical imaging errors of lenses with strong curvatures and a large diameter, resulting in a further increase of the price of the opticals.
U.S. Pat. No. 5,008,743 uses a planar Fresnel plastic lens as a means for achieving telecentric imaging together with a conventional camera and objective, involving a less expensive solution. Due to their optical properties resulting from their discontinuous design, these lenses produce color errors and considerably impaired image quality. Due to the color error, different colors are imaged from the same point of the object on the surface of the CCD cell on mutually different photosensitive measuring elements. This obviously results in a larger measuring error. A measuring error caused by a color error can be corrected with the use of a color filter on the radiation path, so that part of the spectrum is removed by filtering. Nonetheless, filtering reduces the intensity of the radiation emitted by the object, and light sources with higher power must consequently be used. This results in a further increase of the price of the system. A color error can be corrected also by means of programming, as described in this specification. Despite this, the measuring error will be greater than it would be if the object were inspected and measured with appropriately corrected opticals.
U.S. Pat. No. 4,851,698 describes a telecentric imaging arrangement using a row of photosensitive diodes on an image plane on which an image of a strip-like zone of the object is formed with a conventional objective and a strip-like concave mirror with a spherical surface placed between the objective and the object, the objective pupil being placed in the focal plane of the mirror. The solution of this specification also comprises a beam divider in front of the objective, the radiation emitted by a linear light source being reflected through the beam divider and through the strip-like concave spherical mirror to the object. Thus the illumination takes place with the same optical arrangement as the imaging, with the exception that the radiations pass in opposite directions and are separated in the beam divider. In addition, in the arrangement of this specification, the object is placed on a retroreflective substrate for the object to be clearly distinct from the background. The arrangement described here has the major drawback of the imaging errors of the spherical concave mirror, the errors being the same as in lenses, apart from dispersion. Consequently, several lenses must be added to the design described in the specification in order to correct the imaging errors, in a manner similar to lens opticals, whenever good image quality is required, thus resulting in a construction which is at least expensive, although slightly lighter, owing to the larger lens having been replaced with a mirror. What is more, the beam divider used in the solution of the specification is no ideal component, causing part of the radiation from the light source to be dispersed directly from the beam divider through the objective over the photosensitive surface, resulting in turn in a notably lower contrast of the image of the object, and this is obviously the reason why a retroreflective substrate is used in this specification.